1. Field of the Invention
The invention relates generally to the field of wafer or photomask surface inspection, and more particularly, to illumination and light collection optics for inspecting transparent glass substrates.
2. Background Information
Monitoring anomalies, such as pattern defects and particulate contamination, during the manufacture of semiconductor wafers is an important factor in increasing production yields. Numerous types of defects and contamination, especially particles, can occur on a wafer""s surface. Determining the presence, location and type of an anomaly on the wafer surface can aid in both locating process steps at which the anomaly occurred and determining whether a wafer should be discarded.
Originally, particles were monitored manually by visual inspection of wafer surfaces. These particles, usually dust or microscopic silicon particles, caused many of the wafer pattern defects. However, manual inspection proved time-consuming and unreliable due to operator errors or an operator""s inability to observe certain defects.
To decrease the time required to inspect wafer surfaces, many automatic inspection systems were introduced. A substantial majority of these automatic inspection systems detect particles and other anomalies based on the scattering of light. These systems include two major components: illumination optics and collection-detection optics. Illumination optics generally consists of scanning a wafer surface with a source of radiation, e.g., a laser or white light. Particles present on the wafer""s surface scatter incident radiation. The collection optics detect increases in the amount of scattered radiation received, and these increases generally correspond to particles encountered by the illumination optics. This data is reconciled with reference to the known beam position at those moments when the increases in scattered radiation were detected. The data is then converted to electrical signals which can be measured, counted and displayed on a monitor.
Known systems for inspecting wafers that utilize scattered radiation suffer from severe limitations when they are used to inspect transparent articles such as glass mask substrates. One important limitation is that anomalies on transparent substrates generate substantially less scattered radiation than anomalies on non-transparent substrates. There are at least two factors that contribute to this low scattered radiation output. The first is the presence of destructive interference generated between air-side incident and air-side reflected radiation at the surface of the substrate. The second is a substantial reduction in forward scattered radiation that reaches the collection-detection optics.
Forward scattered radiation is radiation that scatters in the same general direction as the radiation from which it originates. For instance, incident radiation that strikes the substrate can generate forward scattered radiation that travels into the substrate. Incident radiation that strikes an anomaly can generate forward scattered radiation that travels past the anomaly and strikes the substrate surface. And radiation that reflects off the substrate surface (reflected radiation) and then strikes an anomaly from below it can generate forward scattered radiation that tends to travel away from the substrate and into the collection-detection optics. Since this last form of forward scattered radiation tends to travel directly into the collection-detection optics, it generally makes up a sizeable portion of the scattered radiation that is collected during a wafer inspection process. Accordingly, the term xe2x80x9cforward scattered radiationxe2x80x9d as used herein refers primarily to forward scattered radiation generated by reflected radiation striking an anomaly from below it.
When a radiation source is directed at the surface of a transparent substrate, very little of the incident radiation reflects off the surface as reflected radiation. This is because a substantial portion of the incident light penetrates into the transparent substrate. In fact, only around 0% to 10% of the incident radiation reflects off the surface. This substantial reduction in reflected radiation off transparent substrates (as compared to silicon wafers) results in a correspondingly substantial reduction in forward scattered radiation off anomalies that is directed at the collection-detection optics.
In addition to these problems, background noise increases on a transparent substrate because incident light penetrates the substrate and then scatters as it hits the chuck used to hold the substrate in position. So this and all of the above factors significantly reduce the signal-to-noise ratio when known systems inspect transparent substrates, resulting in poor detection of particles. Accordingly, there is a need for an inspection system that can produce stronger scattered light signals with higher signal-to-noise ratios when encountering anomalies present on transparent substrates.
The disadvantages and problems associated with inspecting transparent articles such as glass mask substrates have been improved using the present invention.
In accordance with an embodiment of the invention, a method for detecting an anomaly on a first surface of a transparent substrate starts with providing a transparent substrate that has a reflective second surface. The method then comprises directing a radiation beam at the first surface of the substrate so that at least a portion of the radiation penetrates the substrate and strikes the reflective second surface. This radiation is reflected back as a reflected radiation beam through the first surface of the substrate. The method then comprises detecting radiation from the reflected radiation beam. This method can further comprise causing relative motion between the radiation beam and the first surface of the substrate. This method can also further comprise documenting the presence of an anomaly if the detected radiation shows that the reflected radiation beam was scattered upon traversing the first surface.
In accordance with another embodiment, the above method can further comprise directing a second radiation beam at a location on the first surface of the substrate that corresponds to where the reflected radiation beam traverses the first surface, and detecting radiation from the second radiation beam.
In accordance with another embodiment, a method for detecting an anomaly on a first surface of a transparent substrate comprises directing a radiation beam at a second surface of the substrate so that at least a portion of the radiation beam penetrates the substrate and traverses the first surface, and detecting radiation from the radiation beam as it traverses the first surface.
In accordance with another embodiment of the invention, a system for detecting an anomaly on a first surface of a transparent substrate comprises a radiation source operable to emit radiation, an objective operable to focus the radiation into a radiation beam, and a detector mounted to detect radiation. The objective is mounted to direct the radiation beam onto a first location on the first surface of the substrate so that at least a portion of the radiation beam penetrates the substrate and strikes a reflective second surface of the substrate, thereby reflecting the radiation beam back through a second location on the first surface of the substrate.
In accordance other embodiments, the above system can further comprise any one or all of a compensatory plate operable to correct any aberration introduced by the substrate, a collector operable to collect radiation and focus the radiation onto the detector, and/or an optical element operable to redirect the radiation beam to the second location on the first surface of the substrate.
An important technical advantage of the present invention includes reflecting the radiation beam off the reflective second surface of the substrate so that the radiation beam strikes anomalies from the substrate side, rather than from the air side. The use of substrate side radiation increases the sensitivity of the system by reducing radiation loss, reducing interference between scattered and reflected radiation by eliminating collection of the reflected radiation component, reducing background noise, and greatly increasing the amount of forward scattered radiation generated by the system. Another advantage of the invention is that the methods disclosed herein can be performed without significant design changes to current wafer inspection systems and wafer mounting systems.
Other important technical advantages of the present invention are readily apparent to one skilled in the art from the following figures, descriptions, and claims.